Device for separation of sputtered neutrals and high energy ions from sputtered low energy ions

ABSTRACT

A device capable of reducing the background in ion probe mass spectrometry by up to seven orders of magnitude (107) allows separation of sputtered neutrals and high energy sputtered or backscattered ions from low energy sputtered ions. The low energy sputtered ions are fed into a mass analyzer for determination of the mass-to-charge ratio whereas the neutrals and high energy ions which usually produce a high background intensity in the mass spectra are prevented from entering the mass analyzer. This is achieved by positioning the target off-axis with respect to the relevant analyzer axis on the entrance side of the mass analyzer, by placing a shutter with an aperture between target and entrance aperture of the mass analyzer so that sputtered neutrals passing through the shutter aperture impinge on the front panel of the mass analyzer outside the entrance aperture, by producing an electric field between shutter and front panel of the mass analyzer, and by adjusting the direction and strength of the electric field in such a way that low energy sputtered ions passing through the shutter aperture can be deflected into the entrance aperture of the mass analyzer whereas high energy ions are less deflected and thus impinge on the front panel of the mass analyzer outside the entrance aperture.

United States Patent Maul et al.

1 1 Nov. 25, 1975 3,622,781 Liebl .1 250/296 [57] ABSTRACT A devicecapable of reducing the background in ion probe mass spectrometry by upto seven orders of magnitude (10) allows separation of sputteredneutrals and high energy sputtered or backscattered ions from low energysputtered ions. The low energy sputtered ions are fed into a massanalyzer for determination of the mass-to-charge ratio whereas theneutrals and high energy ions which usually produce a high backgroundintensity in the mass spectra are pre vented from entering the massanalyzer. This is achieved by positioning the target off-axis withrespect to the relevant analyzer axis on the entrance side of the massanalyzer, by placing a shutter with an aperture between target andentrance aperture of the mass analyzer so that sputtered neutralspassing through the shutter aperture impinge on the front panel of themass analyzer outside the entrance aperture, by producing an electricfield between shutter and front panel of the mass analyzer, and byadjusting the direction and strength of the electric field in such a waythat low energy sputtered ions passing through the shutter aperture canbe deflected into the entrance aperture of the mass analyzer whereashigh energy ions are less deflected and thus impinge on the front panelof the mass analyzer outside the entrance aperture.

6 Claims, 4 Drawing Figures 1 DEVICE FOR SEPARATION OF SPUTTEREDNEUTRALS AND HIGH ENERGY IONS FROM SPUTTERED LOW ENERGY IONS 175]Inventors: Johann Maul, Westerholzhausen near Munich; Friedrich Schulz;Klaus Wittmaack, both of Munich, all of Germany [73] Assignee:Gesellschaft fur Strahlen-und Umweltforschung mbH, Munich, Neuherberg,Germany {22] Filed: May 2, I973 {21] Appl. No.1356,313

[30] Foreign Application Priority Data Sept. 1, 1972 Germany 2242987[52] US. Cl. 250/296; 250/297; 250/281;

[51] Int. Cl. HOIj 39/00 [58] Field of Search 204/192, 298; 250/281,

[56] References Cited UNITED STATES PATENTS 2,851,608 9/1958 Robinson...250/296 2,852,684 9/1958 Payne 250/281 2,976,413 3/1961 Robinson...250/292 2,995,659 8/1961 Craig 250/296 3,075,076 1/1963 Giinther 250/2923,233,099 2/1966 Berry et al. 250/296 3,487,207 12/1969 Langley 250/281I) i 20d 5 2! us. Patent NOv.25,1975 Sheet10f3 3,922,544

Fig.2

US. Patent Nov. 25, 1975 Sheet 2 of3 3,922,544

ate: 25 9c 5 55::

m w m w w w 0 H m F o W F H 1 7A a 2 20 10 Mass number m/ne US. PatentNov. 25, 1975 Sheet 3 of3 Fig.4

In fensify (relative units) DEVICE FOR SEPARATION OF SPUTTERED NEUTRALSAND HIGH ENERGY IONS FROM SPUTTERED LOW ENERGY IONS BACKGROUND OF THEINVENTION This invention relates to an improvement of ion probe massspectrometers and in particular to those with a quadrupole mass filter.

Ion probe mass spectrometers are now frequently used as a means forstatic (surface) or dynamic (in depth) analysis of the chemicalcomposition of solids. The respective method currently named secondaryion mass spectrometry" (SIMS), has the following physical background:

In a target chamber evacuated by suitable pumps to a chamber pressure ofless than about IO Torr, the tar get surface is bombarded by a primaryion beam produced by a suitable ion gun. the energy usually beingseveral keV. The bombardment results in sputtering of the target. Thesputtered particles are partly ionized (secondary ions) and may thus beanalyzed with respect to their mass-to-charge ratio in a massspectrometer. The prior art has mainly used the method of magnetic massseparation. A review has been given by A. J. Socha published in SurfaceScience 25 (1971) I47. More recently electrical quadrupole filters havebeen applied successfully for mass analysis in ion probe massspectrometers. A description and discussion may be found in a paper byA. Benninghoven and E. Loebach published in The Review of ScientificInstruments 42 (I971) 49.

One problem encountered in constructing an ion probe mass spectrometer,results from the fact that the sputtered particles have an energydistribution with a broad peak at some eV (low energy ions) and a slowlydecreasing tail extending to energies of the order of the primary ionenergy (high energy ions). The high energy ions are the main cause oftrouble with respect to the ion optical lay-out of the apparatus sincethese ions produce tails in the mass lines and thus reduce both the massresolution and the sensitivity in trace analysis.

Additional complications arise in cases where a quadrupole analyzer isused since this type of mass analyzer is only able to filter ions thathave an energy below about 50 eV. Secondary ions and backscatteredprimary ions with an energy above this limit pass through the quadrupoleunfiltered and may enter the ion detector, thus causing a background inthe mass spectrum. Moreover sputtered neutrals entering the quadrupolemay produce ternary ions by means of sputtering internal surfaces or viacollision with residual gas molecules. These ternary ions also passthrough the remainder of the quadrupole unanalyzed and cause a strongbackground in the mass spectrum.

In a very recent paper given by us (K. Wittmaack, J. Maul and F. Schulz,International Journal of Mass Spectrometry and Ion Physics ll (I973)23), we have demonstrated the necessity to prevent sputtered neutralsand high energy sputtered or backscattered ions from entering thequadrupole if one wants to achieve a peak-to-background ratio of betterthan 10.

Accordingly the object of the invention is to add a means to massanalyzers, in particular to quadrupole filters. which allows bothsuppression of all unwanted 2 particles and passage of a high amount ofanalyzablc secondary ions.

SUMMARY OF THE INVENTION In the improvement of ion probe massspectrometers herein presented the target is mounted off-axis withrespect to the relevant analyzer axis on the entrance side of the massanalyzer. between target and entrance aperture of the mass analyzer ashutter with an aperture is arranged in such a way that the straightline defined by the center of the target and the center of shutteraperture crosses the analyzer axis in front of the entrance aperture.Dimension and position of the shutter aperture are arranged in such away that sputtered neutrals passing through the shutter aperture impingeon the front panel of the mass analyzer outside the entrance aperture.Between entrance aperture of the analyzer and shutter aperture adeflecting electric field is produced between electrodes of anyconventional design and fabrication. The direction and strength of theelectric field are adjustable so that the low energy sputtered ionswhich pass through the shutter aperture can be de flected into the massanalyzer whereas the high energy ions are less deflected and impinge onthe front panel of the mass analyzer. The potential of the shutter andthe target with respect to the mass analyzer are adjustable to achieveoptimum operating conditions.

In one embodiment of the invention, the electric field is a static oneproduced between two electrically conductive surfaces. In anotherembodiment the electric field is produced by a plate capacitor.

Using this kind of set-up, one is able to prevent all unwanted particlesfrom entering the mass analyzer. The background intensity in the massspectra may thus be reduced to the electronic noise level of the iondetection system. The gain in sensitivity in trace analysis obtained byadding this set-up to a quadrupole mass filter has been found to amountto 4 to 7 orders of magnitude depending upon the respective experimentalconditions.

Since the present invention is of a very simple design it may be addedto any targetmass analyzer arrangement. The invention provides anoutstanding improvement to ion probe mass spectrometers with quadrupoleanalyzer, it may also be used successfully in combina tion with magneticmass separators.

The features of our invention which are believed to be novel are setforth with particularity in the appended claims. The invention will bemore fully understood from the following detailed description of thearrangement and a presentation of the improvement taken in conjunctionwith the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic representationof a device for separation of sputtered neutrals and high energy ionsfrom sputtered low energy ions.

FIG. 2 is a transverse sectional view of the structure of a device forseparation of sputtered neutrals and high energy ions from sputtered lowenergy ions.

FIG. 3 shows a typical secondary ion mass spectrum obtained uponsputtering an aluminium target using a quadrupole analyzer without ameans to suppress un wanted particles.

FIG. 4 shows the spectrum corresponding to that of FIG. 3, obtainedafter suppression of unwanted parti cles.

Referring to FIG. I, there is shown a quadrupole mass analyzer I with anentrance aperture 2 in a front panel [0. Sputtered particles areproduced by bombardment of a sample 3 with a primary ion beam 4. Thesample 3 is mounted on a target electrode 5, which is rotatable withrespect to the axis normal to the drawing plane and movable parallel tothe direction of primary beam incidence. Between target 3 and aperture 2a shutter electrode 6 with aperture 7 is mounted. Shutter 6 allows onlypart of the sputtered or backscattered particles to enter throughaperture 7 into the region be tween shutter 6 and front panel 10. Thebeam of sec ondary particles emitted from sample 3 and passing throughaperture 7 defines an axis 8 which. by suitably positioning sample 3 andaperture 7, intersects the front panel I at a point 9 outside theentrance aperture 2. Consequently the sputtered neutrals I out of thesecondary beam passing through aperture 7 impinge at point 9 on thefront panel 10 and thus do not enter the mass analyzer I via aperture 2.The electro' static field I1 produced between shutter 7 and front panel[0 by use of suitable electrodes (not shown in FIG. I). is adjusted withrespect to field orientation and field strength so that low energysputtered ions 12 enter into the mass analyzer I via aperture 2 whereashigh energy backscattered or sputtered ions 13 and very low energy ions14 hit front panel 10 outside aperture 2. A special shape of theelectrostatic field 11 is not re quired, yet direction and strength ofthe electrostatic field ll along the path of low energy ions 12 must besuch that these ions pass through aperture 2 parallel to the relevantaxis of the mass analyzer. The electrostatic potential of sample 3,target electrode 5 and shutter electrode 6 may be adjusted with respectto the potential of the mass analyzer so as to achieve optimum lineintensity in the mass spectra.

Referring now to FIG. 2, thee is shown a sectional view of oneembodiment of the inventive device. The primary ion beam 4 hits thesample 3 at an angle 11 with respect to the target normal. The secondarybeam 8 passes through aperture 7 in shutter electrode 6 into a spacedefined by a plate capacitor made up by the couple of plate electrodes[6, 17 The two surfaces l8, l9 ofelectrodes l6, 17 are facing oneanother. Applying a voltage to the plates l6, 17 an electric field isproduced between surfaces [8, 19. as indicated by arrow II. The straightline 20 which passes through the center of aperture 2 is a symmetry axisof the electrodes I6, 17. Line 20 and the straight line 8 whichdesignates the axis of the beam of sputtered neutrals I5 intersect oneanother at an angle 0:. Aperture 2 must lie in the plane of iondeflection which is defined by line 8 and the direction of theelectrostatic field 1]. Shutter 6 with aperture 7 is movable oncapacitor bounding plate elec trode 21 which contains a large opening 22partly masked by shutter 6. Shutter 6 is fixed to plate 21 via theguiding support 23. Plate 21 is mounted to l6, 17 via isolating eyelets24 whereas the plate capacitor is fixed to the front panel [0ofquadrupole I via insulating eyelets 25.

By suitably adjusting direction and strength of the electrostatic fieldII, e.g. about 3 volts/cm in case of the embodiment shown in FIG. 2, lowenergy secondary ions I2 are deflected into aperture 2 whereas highenergy ions 13 and very low energy ions 14 hit the front panel 10outside aperture 2. Optimum operation conditions with respect to a highratio of peak to background intensity in the mass spectra may beobtained by mov ing and rotating target electrode 5 with sample 3, bychanging the angle 01 via moving shutter electrode 6 with aperture 7,and by by fitting the dimension of ap' erture 7 to the dimension of thetarget area investigated.

Referring now to FIGS. 3 and 4, there is demonstrated the reduction inbackground intensity in ion probe mass spectrometry with a quadrupoleanalyzer by means of using a device for separation of sputtered neutralsl5 and high energy ions 13 from sputtered low energy ions 12, inparticular by means of using the embodiment shown in FIG. 2. The resultsshown in FIGS.

3 and 4 were both obtained by bombardment of an aluminium sample with aprimary ion beam 4 of IO kcV Argon ions.

FIG. 3 was obtained without a means to suppress sputtered neutrals I5and high energy ions 13. The sample was placed on axis of the quadrupoleanalyzer 1. One can see that the mass spectrum is disturbed by abackground of high intensity 8, nearly independent of mass number. Theintensity ratio of peak AP to background. P/B is only about 10. This istoo low by several orders of magnitude to allow trace analysis in theparts per million range or below.

FIG. 4 was obtained by using the embodiment shown in FIG. 2. One can seethat a background is no longer observable on a linear scale. Theintensity ratio of peak to background, P/B is now about 10. From acomparison with the results of FIG. 3 one can see that the gain insensitivity obtained by using the inventive device amounts to about 10The reduction in peak intensity at the same time is only about 50%, dueto the low energy dispersion of the device, aimed at in constructing theembodiment of FIG. 2.

Using the inventive device one may thus carry out trace analysis down tothe parts per billion range.

An additional advantage obtained by using the inventive device may bededuced from a comparison of the line shape of the "AF peak in FIGS. 3and 4. One can see that the tail T observed in FIG. 3 has disappeared inFIG. 4. Moreover the full width at half maximum, FWHM of the Al peak isreduced from 0.5 to 0.3 at the same setting of the quadrupoleresolution. Both ef fects are due to a suppression of those sputteredhigh energy ions which have an energy at the upper limit ofdetectability. This additional advantage of the inventive device allowsa determination of trace contaminants with mass numbers in theneighborhood of strong mass lines. e.g. those of the matrix material.

While there has been described in FIG. 2 what is presently consideredthe preferred embodiment of the invention it will be obvious to thoseskilled in the art.

that various changes and modifications may be made therein withoutdeparting from the inventive concept contained therein and it is,therefore, aimed to cover all such changes and modifications as fallwithin the true spirit and scope of the invention.

We claim as our invention:

1. Mass spectrometer apparatus comprising. in combination:

mass analyzer means including a front panel provided with an entranceaperture and defining a longitudinal ion input path passing through saidaperture and extending transverse to said front panel;

means for supporting a target electrode arranged to carry a sample to betested for permitting the samplc to be struck by a primary ion beam at apoint laterally offset from said ion input path for producing particlesby sputtering; t

shutter means presenting a beam-defining aperture for the passage of abeam of such particles from said sample, said beam defining aperturebeing positioned such that the straight line between the point at whichthe primary ion beam strikes the sample and the center of saidbeam-defining aperture intersects said front panel at a point outsidethe region enclosed by said entrance aperture and intersects the linedefining said longitudinal ion input path at a point between said frontpanel and said shutter means; and

means for producing an electric field in the region between said shuttermeans and said front panel to deflect the low energy ions in said beamof particles through said entrance aperture and along said ion inputpath, said electric field producing means including a pair of electrodesdisposed between said front panel and said shutter means and located torespectively opposite sides of the line defining said longitudinal inputpath, said electrodes being spaced from the straight-line path betweenthe center of said beam-defining aperture and the point where said frontpanel is intersected by said straight line between the point where theprimary ion beam strikes the sample and the center of said 6beamdefining aperture. whereby neutral particles and high energy ionspassing through said beamdefining aperture strike said front paneloutside the region enclosed by said entrance aperture and are thusprevented from entering said mass analyzer means.

2. An arrangement as defined in claim 1 wherein said pair of electrodesconstitute plate electrodes of a plate capacitor.

3. An arrangement as defined in claim I further comprising mountingmeans supporting at least one of the following: said shutter means andsaid target electrode supporting means. for movement in a directiontransverse to said longitudinal ion input path for varying the anglebetween said longitudinal ion path and the straight line between thecenter of said bearn-dcfining aperture and the point at which theprimary ion beam strikes the sample.

4. An arrangement as defined in claim 1 wherein at least one of thefollowing: said sample, said target elec trode, and said shutter meansis arranged to be placed at an electrostatic potential relative to thepotential of said mass analyzer for optimizing the line intensity in themass spectra produced at the output of said mass analyzer.

5. An arrangement as defined in claim 1 wherein said mass analyzer meansis a quadrupole mass analyzer.

6. The device of claim 1 wherein the electric field is a static one.

1. Mass spectrometer apparatus comprising, in combination: mass analyzermeans including a front panel provided with an entrance aperture anddefining a longitudinal ion input path passing through said aperture andextending transverse to said front panel; means for supporting a targetelectrode arranged to carry a sample to be tested for permitting thesample to be struck by a primary ion beam at a point laterally offsetfrom said ion input path for producing particles by sputtering; shuttermeans presenting a beam-defining aperture for the passagE of a beam ofsuch particles from said sample, said beam defining aperture beingpositioned such that the straight line between the point at which theprimary ion beam strikes the sample and the center of said beam-definingaperture intersects said front panel at a point outside the regionenclosed by said entrance aperture and intersects the line defining saidlongitudinal ion input path at a point between said front panel and saidshutter means; and means for producing an electric field in the regionbetween said shutter means and said front panel to deflect the lowenergy ions in said beam of particles through said entrance aperture andalong said ion input path, said electric field producing means includinga pair of electrodes disposed between said front panel and said shuttermeans and located to respectively opposite sides of the line definingsaid longitudinal input path, said electrodes being spaced from thestraight-line path between the center of said beam-defining aperture andthe point where said front panel is intersected by said straight linebetween the point where the primary ion beam strikes the sample and thecenter of said beam-defining aperture, whereby neutral particles andhigh energy ions passing through said beamdefining aperture strike saidfront panel outside the region enclosed by said entrance aperture andare thus prevented from entering said mass analyzer means.
 2. Anarrangement as defined in claim 1 wherein said pair of electrodesconstitute plate electrodes of a plate capacitor.
 3. An arrangement asdefined in claim 1 further comprising mounting means supporting at leastone of the following: said shutter means and said target electrodesupporting means, for movement in a direction transverse to saidlongitudinal ion input path for varying the angle between saidlongitudinal ion path and the straight line between the center of saidbeam-defining aperture and the point at which the primary ion beamstrikes the sample.
 4. An arrangement as defined in claim 1 wherein atleast one of the following: said sample, said target electrode, and saidshutter means is arranged to be placed at an electrostatic potentialrelative to the potential of said mass analyzer for optimizing the lineintensity in the mass spectra produced at the output of said massanalyzer.
 5. An arrangement as defined in claim 1 wherein said massanalyzer means is a quadrupole mass analyzer.
 6. The device of claim 1wherein the electric field is a static one.